An Al-based alloy, being low in the electrical resistivity and easy to process, is widely used in a field of flat panel displays (FPDs) such as liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescent displays (ELDs), field emission displays (FEDs) and MEMS (Micro Electro Mechanical System) displays, touch panels and electronic papers and is used as materials for interconnection films, electrode films and reflective electrode films.
For instance, an active matrix type liquid crystal display includes a thin film transistor (TFT) that is a switching element, a pixel electrode constituted of a conductive oxide film and a TFT substrate having an interconnection containing a scanning line and a signal line. As an interconnection material that constitutes the scanning line and signal line, generally, thin films made of a pure Al or an Al—Nd alloy are used. However, when various kinds of electrode portions formed by the thin films are directly connected to a pixel electrode, insulating aluminum oxide is formed at an interface to increase the contact electrical resistance. Accordingly, so far, a barrier metal layer made of a refractory metal such as Mo, Cr, Ti or W has been disposed between the Al interconnection material and the pixel electrode to reduce the contact electrical resistance.
However, in a method of interposing a barrier metal layer such as mentioned above, there is a problem in that a production process becomes troublesome to be high in the production cost.
In this connection, the inventors have proposed, as a technology that, without interposing a barrier metal layer, enables to directly connect an electroconductive oxide film that constitutes a pixel electrode and an interconnection material (direct contact technology), a method in which as an interconnection material a thin film of an Al—Ni alloy or an Al—Ni alloy further containing a rare earth element such as Nd or Y is used (see, JP-A-2004-214606). When the Al—Ni alloy is used, at an interface, electroconductive Ni-containing precipitates are formed to suppress insulating aluminum oxide is inhibited from generating; accordingly, the contact electrical resistance can be suppressed low. Furthermore, when the Al—Ni-rare earth element alloy is used, the heat resistance can be further improved.
Now, when an Al-based alloy thin film is formed, in general, a sputtering process that uses a sputtering target is adopted. According to the sputtering method, plasma discharge is generated between a substrate and a sputtering target (target material) constituted of a material same as that of the thin film, a gas ionized by the plasma discharge is brought into collision with the target material to knock out atoms of the target material to deposit on the substrate to produce a thin film. The sputtering method, different from a vacuum deposition method, has an advantage in that a thin film having a composition same as that of the target material can be formed. In particular, an Al-based alloy thin film deposited by use of the sputtering method can dissolve an alloy element such as Nd that cannot be dissolved in an equilibrium state and thereby can exert excellent performance as a thin film; accordingly, the sputtering method is an industrially effective thin film producing method and a development of a sputtering target material that is a raw material thereof is forwarded.
Recently, in order to cope with the productivity enlargement of FPDs, a depositing rate at the sputtering step tends to be increased more than ever. In order to increase the depositing rate, the sputtering power can be most conveniently increased. However, when the sputtering power is increased, sputtering defects such as splashes (fine melt particles) are caused to generate defects in the interconnection film; accordingly, harmful effects such as deterioration the yield and operation performance of the FPDs are caused.
In this connection, in order to inhibit the splashes from occurring, for instance, methods described in JP-A-10-147860, JP-A-10-199830, JP-A-11-293454 and JP-A-2001-279433 have been proposed. Among these, in all of JP-A-10-147860, JP-A-10-199830 and JP-A-11-293454 that are based on the viewpoint in that the splashes are caused owing to fine voids in a target material texture, a dispersion state of particles of a compound of Al and a rare earth element in an Al matrix is controlled (JP-A-10-147860), a dispersion state of a compound of Al and a transition metal element in an Al matrix is controlled (JP-A-10-199830) or a dispersion state of an intermetallic compound between an additive element and Al in a target is controlled (JP-A-11-293454) to inhibit the splashes from occurring. Furthermore, JP-A-2001-279433 discloses a technology in which, in order to reduce the arching (irregular discharge) that is a cause of the splashes, the hardness of a sputtering surface is controlled, followed by applying finish working to inhibit surface defects due to the machining from occurring.
On the other hand, a technology that inhibits a target from warping due to heating at the production of mainly a large target has been disclosed (JP-A-2006-225687). In JP-A-2006-225687, it is disclosed that, with an Al—Ni-rare earth element alloy sputtering target as an object, when more than a predetermined number of compounds having an aspect ratio of 2.5 or more and a circle equivalent diameter of 0.2 μm or more are present in a cross section vertical to a target plane, the target can be inhibited from deforming.
As mentioned above, although various technologies for reducing the generation of the splashes to reduce the sputtering defects have been proposed, a further improvement has been demanded. In particular, the initial splashes occurring in an initial stage of the sputtering deteriorate the yield of FPDs and thereby cause a serious problem. However, the splash inhibition technologies disclosed in JP-A-10-147860, JP-A-10-199830, JP-A-11-293454 and JP-A-2001-279433 cannot sufficiently effectively inhibit the initial splashes from occurring. Furthermore, among Al-based alloys, in an Al-based alloy sputtering target that is useful as an interconnection material capable of directly connectable with a elctroconductive oxide film that constitutes a pixel electrode and is used to form a thin film of an Al—Ni—La—Si system Al-based alloy that is applicable as an interconnection material capable of directly contactable with a semiconductor layer of a thin film transistor, a technology that can overcome the above-mentioned problems has not yet been proposed.
The inventors have made intensive studies to provide an Al-based alloy sputtering target that can reduce the splashes generated during the sputtering deposition, in particular, the initial splashes generated at the initial stage of the sputtering deposition.
As a result, it was found that both of particle size distributions of intermetallic compounds (an Al—Ni system intermetallic compound mainly composed of Al and Ni and an Al—La system intermetallic compound mainly composed of Al and La) contained in an Al—Ni—La system Al-based alloy sputtering target have a significant correlation with the generation of the initial splashes; accordingly, when the particle size distributions of the intermetallic compounds are appropriately controlled, an expected object can be achieved, and this was previously filed as a patent application (Japanese patent application No. 2006-313506). In what follows, the above-mentioned invention is called as “precedent Al—Ni—La system Al-based sputtering target” or simply as “the precedent invention” in some cases.